Implemented WebVTTToMarkdown demo

Also implemented StringBuilder::insert() and StringBuilder::replaceAll(),
along with some helper functions, in order to support the demo application.

No tests for this stuff yet, I'm afraid.
Improve test-coverage for StringBuilder

There were some bugs in some of the StringBuilder routines
that were added in, I've tried to get them as sturdy as possible
to be sure that my demo will always work correctly.
Speed up StringBuilder.ReplaceAll tests a bit
Merge pull request #22 from caitp/issue-22

List<T> should support appending other lists.